1 INTRODUCTION
4.3 Oxidation of MAA
The energy efficiency of MAA oxidation was observed to be less dependent on pulse repetition frequency than with oxidation of oxalate. However, since the substance was observed to be readily degradable with PCD treatment, the indifference in pulse frequency may indicate that the majority of the oxidants produced by a single pulse are immediately consumed. This makes the amount of oxidants produced by a single pulse a limiting factor to the oxidation process. Efficiency for 30 % removal of MAA, which all experiments well reached, is presented with different operating parameters in Figure 21.
CNO3- = 11,843E + 8,055
CNO3- = 11,212E + 4,6443
0 50 100 150 200 250 300 350
0 5 10 15 20 25 30
Nitrate concentration, mg/L
Energy delivered, kWh/m3
NC TiO2 Linear (NC) Linear (TiO2)
Fig ure 21. Oxidation of 100 ppm an d 300 ppm MAA solution s with pulse fr equen cies o f 833 pps an d 500 pps. 1: n on -catalytic treatment, 2: catalytic treatmen t.
Fig ure 22. Oxidation of MAA with in itial con cen tr ation s of 100 ppm (above) an d 300 ppm (below). Th e values repr esen t aver age r esults of r epeated exper imen ts.
0,00
Comparison between the results of non-catalytic and catalytic experiments is presented in Figures 21 and 22. One can see in Figure 22 that the curves resulting from different frequencies are nearly identical, but that the lower frequency curves have a tendency of settling just below those of higher frequency. For this reason, there is probably some, yet very fine contribution of the longer-living oxidants that have more time to react between pulses. From Figure 21, one can see that the energy efficiency for 30-% degradation of MAA is increased with higher initial concentration. This is explained by the probability of oxidant and pollutant collision, which is increased with higher substrate concentration.
Similarly to the results obtained from oxalate degradation, the applied catalyst cannot be observed to attribute any improvement to the process. Indeed, as one can see from Figure 21, the poorer efficiency of catalytic treatment against non-catalytic for 30-% degradation of the target substance is even clearer with MAA than with oxalate. For the initial concentration of 300 ppm, the catalytic oxidation at 833 pps experienced lag during the first kWh/m3 delivered, which is clearly visible in Figure 22. The slow oxidation rate in the beginning logically resulted in poor efficiency determined for the 30-% reduction of MAA with the used parameters. From the comparison to the non-catalytic reference (Fig. 21) one can see that the efficiency for the non-catalytic oxidation of 300 ppm MAA solution at 833 pps is over twice that of the catalytic one. This may be due to problems that were contemplated earlier in chapter 4.2 regarding the catalyst presence.
5 CONCLUSIONS
Degradation of aqueous oxalate was expectedly slow, achieving 3.9 g/kWh efficiency for 30-% degradation at best in alkaline solution at 500 pps. However, at higher frequency of 833 pps, better oxidation rate was observed with acidic than alkaline solution as respective 80-% and 50-% oxidation rates were achieved. This is because for some, so far inexplicable reason, the oxidation in alkaline solutions with 833 pps seized after 12.5 kWh/m3 of energy was delivered, remaining at 50-% level. To explain this phenomenon, further studies are required on the effect of pH to the process, and its complex interaction with other process parameters. Overall oxidation efficiency of oxalate showed its dependence on pulse frequency, giving clearly better overall results with lower frequency.
This is considered to be mostly due to the contribution of longer-living oxidants, like ozone, that have more time to react with the target pollutants between the discharge pulses, during which short-living species like hydroxyl radical prevail.
Degradation of aqueous MAA with PCD is overall efficient. Highest efficiency of 50 g/kWh for 30-% removal was achieved with 300 ppm concentration of the substrate at low pulse repetition frequency of 500 pps. Overall results with MAA indicated that the oxidation efficiency of the substance shows little, yet some dependence on pulse repetition frequency as lower frequency resulted in somewhat better efficiency. Meglumine acridone acetate is therefore suggested to be readily degradable along introduction of both powerful short-living oxidants and ozone that are rapidly consumed. From a practical aspect, treatment of water containing compounds like fast-reacting MAA could be considered to benefit from high pulse repetition frequency: the convenience of shorter treatment time over the modest improvement in energy efficiency might prove remarkable. Further studies for PCD use of the present configuration in field conditions and with real life solutions with e.g. industrial origin, are therefore recommended.
With both oxalate and MAA, pH of alkaline solutions decreased to acidic during treatment.
This is considered to be the result of nitric acid and nitrates that were produced in the process from atmospheric nitrogen. The yield for nitrate formation showed small but consistent difference for non-catalytic and catalytic treatment with respective 11.8 g/kWh and 11.2 g/kWh rates. Further studies are required to explain the mechanism and possible catalytic effect resulting in the different yields.
Commercial TiO2 photocatalyst Degussa P25 that was used to coat the grounded plate electrodes of the PCD device attributed no enhancement in oxidation rates of either oxalate or MAA. Moreover, in several occasions the results for catalytic treatment indicated lower efficiency than the corresponding non-catalytic ones, which may, regarding the accuracy of measurements, be explained by the alteration of discharge conditions by the presence of the catalyst. Ultra-violet radiation may also not be sufficient in the experimental conditions to attribute notable photocatalytic activity. Further studies are recommended for different ways to introduce the catalyst to the present configuration, e.g. means to locate the catalyst
directly into the plasma field. Additional areas for further studies could include possible combinations of PCD with different water treatment methods, electrode material abrasion, and residual toxicity or secondary pollution.
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Fig ure A1. pH devel opmen t in oxalate solution s dur in g n on -catalytic treatmen t.
Fig ure A3. Non -catalytic oxalate oxidation . Initial pH values of 3.5 (low) and 10 (high ) an d th e pulse fr equen cies of 833 pps an d 500 pps. 1s t an d 2n d stand for fir st an d r epeated exper imen ts un der same con dition s.
Fig ure A4. Catalytic oxalate oxidation . Initial pH values of 3.5 (low) and 10 (h igh ) an d th e pulse fr equen cies of 833 pps an d 500 pps. 1s t an d 2n d stan d for fir st an d r epeated